Grease composition and mechanical part

ABSTRACT

The invention provides a grease composition containing a base oil, a thickener and an additive, characterized in that the base oil has a kinetic viscosity at 40° C. of 50 to 200 mm 2 /s; the thickener contains a diurea compound represented by formula (I): R 1 —NHCONH—R 2 —NHCONH—R 3  (wherein R 2  is a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms, and R 1  and R 3 , which may be the same or different, represent cyclohexyl group or a straight-chain or branched alkyl group having 8 to 22 carbon atoms); and the additive is at least one selected from the group consisting of a metal salt of naphthenic acid, a salt of a fatty acid having 6 to 10 carbon atoms and an aliphatic amine having 6 to 10 carbon atoms, and a metal salt of an organic sulfonic acid. The composition of the invention shows excellent anti-fretting properties under the conditions of ordinary temperature to low temperature (e.g., −50° C., preferably about −40° C.).

TECHNICAL FIELD

The present invention relates to a grease composition and a mechanical part. More particularly, the present invention relates to a lubricating grease composition excellent in the anti-fretting properties under the conditions from ordinary temperature to low temperature, and a mechanical part, such as a bearing, containing the grease composition therein.

BACKGROUND ART

Motorcars, electrical machines, and a variety of mechanical parts and products are often transported by train or truck. During the transportation, fretting may occur on the grease-applied lubricating parts due to vibration induced by rail joints and rough roads. The fretting is a surface damage induced under a minute amplitude of vibration, and generates oxidized debris in the air, which is said to often cause abrasive action to lead to considerable abrasion (Yuji Yamamoto et al., “Tribology” Rikogakusha Publishing Co., Ltd. (25 Nov. 1999): 201-203).

For prevention of the occurrence of fretting, there are some proposals, for example, (1) to decrease the amplitude of the relative sliding motion, (2) to separate both contact surfaces to prevent direct contact, and (3) to prevent the adhesion of the surfaces by coating the contact surfaces with a phosphate coating or the like or supplying a lubricating oil or grease (Yuji Yamamoto et al., “Tribology” Rikogakusha Publishing Co., Ltd. (25 Nov. 1999): 201-203).

In fact, many greases with excellent anti-fretting properties have been developed and examined mostly in the technical field of grease used in the wheel bearings of cars. For example, it is reported that when compared with a grease where a lithium soap is conventionally used as a thickener, a grease employing an aromatic diurea as a thickener to increase the consistency exhibits significantly improved anti-fretting properties (T. Endo: Eurogrease, November/December (1997): 25-40).

Practically, the improvement in grease flowability has been made to solve the problem of fretting, as mentioned above, by changing the kinds of thickeners, increasing the consistency, and the like.

However, the aromatic diurea has poor flowability. Therefore, when the aromatic diurea is used for bearings, a short lubricating life is exhibited in the angular contact ball bearings due to insufficient flow of the grease into lubrication parts, although a long life is exhibited in the deep-groove ball bearings. The application of the aromatic diurea to bearings is thus limited. As mentioned above, the improvement of grease by changing the thickeners induces the problem that the resultant greases are lacking in versatility.

In addition, the increase in grease consistency leads to the problem of leakage, so that such greases cannot be used for the parts sensitive to leakage. This also results in lack of versatility.

Any of the above-mentioned measures are taken against the fretting which may occur at ordinary temperature, and there is almost no report that the fretting can be prevented under the conditions of low temperature.

Under the conditions of low temperature, the flow of grease is lowered as the viscosity of the base oil contained in the grease increases, so that the ability to prevent the adhesion of the surfaces by supplying a grease thereto as mentioned in the above item (3) cannot be expected. As a measure to control the increase in viscosity of the base oil at low temperature, it is proposed to use a base oil mainly comprising expensive synthetic oils. However, the above-mentioned measure has the drawbacks that the effect of controlling the increase in viscosity of base oil is insufficient and the raw material cost becomes high.

SUMMARY OF INVENTION Technical Problem

An object of the invention is to provide a grease composition excellent in anti-fretting properties.

Another object is to provide a grease composition capable of exhibiting excellent anti-fretting properties under the conditions from ordinary temperature to low temperature (for example, at −50° C., preferably about −40° C.), without the use of a base oil mainly comprising expensive synthetic oils.

A further object of the invention is to provide a mechanical part where the above-mentioned grease composition is used.

Solution to Problem

The invention provides a grease composition and a mechanical part as shown below.

(1) A grease composition comprising a base oil, a thickener and an additive, wherein:

the base oil has a kinetic viscosity at 40° C. of 50 to 200 mm²/s;

the thickener comprises a diurea compound represented by the following formula (I):

R¹—NHCONH—R²—NHCONH—R³  (I)

(wherein R² is a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms, and R¹ and R³, which may be the same or different, represent cyclohexyl group or a straight-chain or branched alkyl group having 8 to 22 carbon atoms); and

the additive is at least one selected from the group consisting of a naphthenic acid metallic salt, a salt of a fatty acid having 6 to 10 carbon atoms and an aliphatic amine having 6 to 10 carbon atoms, and a metal salt of an organic sulfonic acid.

(2) The grease composition described in the above-mentioned (1), wherein the naphthenic acid metallic salt is at least one selected from the group consisting of zinc cyclopentanoate and zinc cyclohexanoate; the amine salt is a salt of a straight-chain fatty acid having 6 to 10 carbon atoms and a straight-chain aliphatic amine having 6 to 10 carbon atoms; and the metal salt of an organic sulfonic acid is at least one selected from the group consisting of calcium dinonylnaphthalenesulfonate, zinc dinonylnaphthalenesulfonate and barium dinonylnaphthalenesulfonate.

(3) The grease composition described in the above-mentioned (1) or (2), wherein the naphthenic acid metallic salt is contained in an amount of 0.5 to 5.0 mass %, the amine salt is contained in an amount of 1.5 to 5.0 mass %, and the metal salt of an organic sulfonic acid is contained in an amount of 1.5 to 5.0 mass %.

(4) A mechanical part in which the grease composition described in any one of the above-mentioned (1) to (3) is packed.

Advantageous Effects of Invention

The grease composition of the invention has excellent anti-fretting properties. Therefore, the grease composition of the invention can be used for lubrication of various mechanical parts. The targeted mechanical parts are not particularly limited, but include rolling bearings, ball screws, linear motion guide bearings, a variety of gears, cams and constant-velocity joints, journal bearings (plain bearings), pistons, screws, ropes, chains and the like. In particular, the grease composition of the invention is suitable for the rolling bearings.

DESCRIPTION OF EMBODIMENTS

The base oil used in the grease composition of the invention is not particularly limited so long as the base oil has a kinetic viscosity at 40° C. of 50 to 200 mm²/s. When the kinetic viscosity at 40° C. is less than 50 mm²/s, the resultant grease film tends to be thinner and a sufficient thickness of the grease film may not be ensured. When the kinetic viscosity at 40° C. exceeds 200 mm²/s, the viscosity resistance is apt to increase.

From the viewpoints of cost and performance, mineral oils are most preferable. The mineral oils or synthetic oils may be used in combination when necessary.

As the synthetic oils, ester-based synthetic oils including diester oils and polyol ester oils; synthetic hydrocarbon oils including poly α-olefin and polybutene; ether-based oils including alkyldiphenyl ethers and polypropylene glycols; silicone oils; and the like can be used. Those base oils can be used alone or in combination.

In consideration of the cost and performance, the content of the mineral oil in the base oil may preferably be 70 mass % or more, more preferably 80 to 100 mass %, and most preferably 100 mass %.

The thickener used in the grease composition of the invention comprises a particular diurea compound represented by general formula (I).

The diurea compound may be contained in the grease composition of the invention in such an amount that the consistency of the grease composition is preferably in the range of 235 to 325, more preferably 280 to 325. To be more specific, the diurea compound may preferably be contained in an amount of 5 to 15 mass %, more preferably 8 to 12 mass %, based on the total mass of the grease composition.

When the content of the diurea compound is less than 5 mass %, the grease tends to soften and leak from the mechanical part upon application of a shear force to the grease. The content of 15 mass % or more may lower the flowability of grease.

In contrast to this, it is impossible to impart sufficient anti-fretting properties to the grease where a metallic soap such as lithium soap is used as the thickener.

The additive used in the grease composition of the invention comprises at least one selected from the group consisting of a metal salt of naphthenic acid, a salt of a fatty acid having 6 to 10 carbon atoms and an aliphatic amine having 6 to 10 carbon atoms, and a metal salt of an organic sulfonic acid.

Examples of metals for the metal salt of naphthenic acid include zinc, magnesium, calcium, aluminum and the like. In particular, zinc salts are most advantageous.

The fatty acid having 6 to 10 carbon atoms for constituting the above-mentioned amine salt may be a straight-chain or branched fatty acid.

The aliphatic amine having 6 to 10 carbon atoms for constituting the above-mentioned amine salt may also be a straight-chain or branched aliphatic amine. The amine salt from a straight-chain fatty acid having 6 to 10 carbon atoms and a straight-chain aliphatic amine having 6 to 10 carbon atoms is preferred, and the amine salt from a straight-chain fatty acid having 8 carbon atoms and a straight-chain aliphatic amine having 8 carbon atoms is further preferred.

Examples of the organic sulfonic acid for constituting the above-mentioned metal salt of an organic sulfonic acid include synthetic sulfonic acids such as petroleum sulfonate, dinonylnaphthalenesulfonic acid, heavy alkylbenzenesulfonic acid and the like, which are obtainable by sulfonating an aromatic hydrocarbon component contained in the lubricating oil fraction.

Examples of metals for the above-mentioned metal salt of an organic sulfonic acid include calcium, zinc, barium, sodium and the like. The calcium salt of an organic sulfonic acid may be a neutral or highly basic salt. For example, calcium salts having a base number of about 50 to 500 mgKOH/g can be used.

The metallic salts of dinonylnaphthalenesulfonic acid are preferable, and in particular, calcium salts, zinc salts and barium salts of dinonylnaphthalenesulfonic acid are preferred.

In the grease composition of the invention, the content of the metal salt of naphthenic acid may preferably be 0.5 to 5.0 mass %, and more preferably 1.0 to 5.0 mass %.

The content of the salt obtained from the fatty acid having 6 to 10 carbon atoms and the aliphatic amine having 6 to 10 carbon atoms may preferably be 1.5 to 5.0 mass %, and more preferably 2.0 to 5.0 mass %.

The content of the metal salt of an organic sulfonic acid may preferably be 1.5 to 5.0 mass %, and more preferably 2.0 to 5.0 mass %.

When two or more kinds of salts are used in combination, the respective contents may appropriately be adjusted.

When the content is less than the above-mentioned lower limit, the effect of preventing the fretting at ordinary temperature or low temperature is unsatisfactory although there appears a rust inhibitory effect. The content of more than 5.0 mass % is considered to be uneconomical because more effect cannot be recognized.

The metal salt of naphthenic acid, the fatty acid amine salt, and the organic sulfonic acid salt used as the additives in the grease composition of the invention are typical rust inhibitors for lubricating oil, which are used in wide applications, also as the rust inhibitors for the grease compositions.

However, there is neither report nor disclosure to the effect that the above-mentioned metal salt of naphthenic acids, the amine salts, and the metal salt of an organic sulfonic acids have the effect on prevention of the fretting damage under the conditions of ordinary temperature to low temperature.

When necessary, the grease composition of the invention may further comprise other general-purpose additives in addition to the above-mentioned metal salt of naphthenic acid, amine salt, and metal salt of an organic sulfonic acid. For example, the following additives can be used:

-   -   Antioxidant

Amines: phenyl α-naphthylamine, alkylated phenyl α-naphthylamine, alkylated diphenylamine and the like.

Phenols: hindered phenols such as 2,6-di-tert-butyl-p-cresol, pentaerythrityl tetrakis [3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate], octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate and the like.

Quinolines: 2,2,4-trimethyl-1,2-dihydroquinoline polymer and the like.

-   -   Rust Preventive

Carboxylic acids and derivatives thereof; alkenylsuccinic anhydride, alkenylsuccinate, half ester of alkenylsuccinate.

Carboxylates; metal (Ca, Ba, Mg, Al, Zn, Na or the like) salts of fatty acids, dibasic acids, lanolin fatty acids, alkenylsuccinic acids and the like, or amine salts thereof.

Passivators; sodium nitrite, sodium molybdate and the like.

Esters; sorbitan trioleate, sorbitan monoleate and the like.

Metal corrosion inhibitors; benzotriazole or derivatives thereof, zinc oxide and the like.

-   -   Extreme Pressure Agent

Phosphorus-containing compounds; tricresyl phosphate, tri-2-ethylhexyl phosphate and the like

Sulfur-containing compounds; dibenzyl disulfide, a variety of polysulfides and the like.

Sulfur-phosphorus containing compounds; triphenyl phosphorothionate

Organic metal-based extreme pressure agents; Zn, Mo, Sb, Bi salt or the like of dialkyl dithiophosphate, Zn, Mo, Sb, Ni, Cu, Bi salt or the like of dialkyl dithiocarbamate, and the like.

Others, e.g., ashless dithiocarbamate, ashless dithiophosphate carbamate and the like.

-   -   Solid Lubricant: Molybdenum Disulfide, Graphite, PTFE, MCA and         the Like.

Under the conditions of ordinary temperature, it is possible to prevent the fretting by changing the kinds of thickeners (the effect as mentioned in the above-mentioned item (2) obtainable by the aromatic urea coating), and increasing the consistency (i.e., improving the flow of grease), as shown in Comparative Examples 6 to 10, 19 and 20 to be described later.

At low temperature, however, flowability of the grease is lowered, so that the effect of preventing the fretting by the presence of grease cannot be expected.

The grease composition of the invention comprises a particular additive (a metal salt of naphthenic acid, a salt of a fatty acid having 6 to 10 carbon atoms and an aliphatic amine having 6 to 10 carbon atoms, or a metal salt of an organic sulfonic acid).

Therefore, for example, when the grease composition of the invention is used for bearings, the strong adsorption of the above-mentioned additive to the surface of members induces the adhesive action at the interface between the diurea grease and the balls and between the diurea grease and the inner/outer ring races. Even under the conditions where the flow of grease is lowered, both surfaces can be sufficiently separated to avoid the direct contact, which is supposed to prevent the fretting at low temperature.

Those skilled in the art could not easily expect the effects according to the invention of improving the anti-fretting properties under the conditions from ordinary temperature to the low temperature by the addition of the particular additive.

EXAMPLES 1 TO 14 AND COMPARATIVE EXAMPLES 1 TO 17

The predetermined amounts of raw materials isocyanate and amine were allowed to react at a molar ratio of 1 to 2 in the base oil. After a given amount of additives was added, the resultant mixture was adjusted to have a predetermined consistency using a three-roll mill.

Comparative Example 18

In a predetermined amount of base oil, lithium hydroxy stearate was dissolved under application of heat with stirring. After cooling the mixture, a given amount of additives was added, and the resultant mixture was adjusted to have a predetermined consistency using a three-roll mill.

<Test Methods>

-   -   Worked penetration: JIS K 2220.7     -   Fafnir friction oxidation test (in accordance with ADTM D 4170)     -   Specification test for evaluating the anti-fretting properties

A test grease was applied to two sets of test thrust bearings as shown below, and then the bearings were subjected to a prescribed oscillation motion to determine the abrasion wear (weight loss by the fretting wear).

[Test Conditions]

Bearing: ANDREWS W ⅝ Load: 2450 N (550 lbf) (contact pressure: 1861 MPa) Angle of oscillation: 0.21 rad (12°) Oscillation cycle: 25 Hz Duration: 22 hours

Temperatures: 25° C. and −30° C.

Filling amount of grease: 1.0 g per bearing Abrasion wear: weight loss of the race per bearing (by dividing the total weight loss of the test bearing races by 2)

[Judgment]

oo: abrasion wear of 2.0 mg or less (anti-fretting properties: excellent) o: abrasion wear of more than 2.0 mg and less than 5.0 mg (anti-fretting properties: good) Δ: abrasion wear of more than 5.0 mg and less than 15.0 mg (anti-fretting properties: slightly present) x: abrasion wear of 15.0 mg or more (anti-fretting properties: absent) Test for determining corrosion preventive properties: ASTM D1743-73 (test for determining corrosion preventive properties for bearings)

[Judgment]

o: absence of corrosion x: presence of corrosion

The results are shown in Tables 1 to 5.

TABLE 1 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Isocyanate MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ Amine ODA(1) ODA(1) ODA(1) ODA(1) ODA(1) ODA(1) ODA(1) (molar ratio) CHA(7) CHA(7) CHA(7) CHA(7) CHA(7) CHA(7) CHA(7) Thickener 11 11 11 11 11 11 11 (mass %) Base oil (mixing Mineral Mineral Mineral Mineral Mineral Mineral Mineral ratio by mass) oil (100) oil (100) oil (100) oil (100) oil (100) oil (100) oil (100) Base oil (mass %) 88.3 88.0 87.0 87.0 87.0 87.0 87.0 Kinetic viscosity 100 100 100 100 100 100 100 of base oil (40° C.: mm²/s) Zinc 0.7 1.0 2.0 naphthenate*² Fatty acid 2.0 amine salt*³ Ca sulfonate*⁴ 2.0 Zn sulfonate*⁵ 2.0 Ba sulfonate*⁶ 2.0 Worked 300 300 300 300 300 300 300 consistency Abrasion wear ∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ (25° C.) Abrasion wear ∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ (−30° C.) Corrosion ∘ ∘ ∘ ∘ ∘ ∘ ∘ preventive properties

TABLE 2 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex.12 Ex. 13 Ex. 14 Isocyanate MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ Amine OA OA OA OA OA OA OA (molar ratio) Thickener 9 9 9 9 9 9 9 (mass %) Base oil (mixing Mineral Mineral Mineral Mineral Mineral Mineral Mineral ratio by mass) oil (100) oil (100) oil (100) oil (100) oil (100) oil (100) oil (100) Base oil (mass %) 90.3 90.0 89.0 89.0 89.0 89.0 89.0 Kinetic viscosity 100 100 100 100 100 100 100 of base oil (40° C.: mm²/s) Zinc 0.7 1.0 2.0 naphthenate*² Fatty acid 2.0 amine salt*³ Ca sulfonate*⁴ 2.0 Zn sulfonate*⁵ 2.0 Ba sulfonate*⁶ 2.0 Worked 300 300 300 300 300 300 300 consistency Abrasion ∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ wear (25° C.) Abrasion ∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ ∘∘ wear (−30° C.) Corrosion ∘ ∘ ∘ ∘ ∘ ∘ ∘ preventive properties

TABLE 3 Comp. Comp. Comp. Comp. Comp. Comp. Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Isocyanate MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ Amine ODA(1) ODA(1) ODA(1) ODA(1) ODA(1) ODA(1) (molar ratio) CHA(7) CHA(7) CHA(7) CHA(7) CHA(7) CHA(7) Thickener 11 11 11 11 11 11 (mass %) Base oil (mixing Mineral Mineral Mineral Mineral Mineral Mineral ratio by mass) oil (100) oil (100) oil (100) oil (100) oil (100) oil (100) Base oil (mass %) 89.0 87.0 87.0 87.0 87.0 87.0 Kinetic viscosity 100 100 100 100 100 100 of base oil (40° C.: mm²/s) Phosphoro- 2.0 thionate*⁷ Calcium 2.0 carbonate*⁸ Zinc stearate*⁹ 2.0 MoDTC*¹⁰ 2.0 ZnDTP*¹¹ 2.0 Worked 300 300 300 300 300 300 consistency Abrasion wear x ∘ x ∘ x x (25° C.) Abrasion wear x x x x x x (−30° C.) Corrosion x x x x x x preventive properties

TABLE 4 Comp. Comp. Comp. Comp. Comp. Comp. Ex. 7 Ex. 8 Ex. 9 Ex. 10 Ex. 11 Ex. 12 Isocyanate MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ Amine ODA(1) ODA(1) ODA(1) OA OA OA (molar ratio) CHA(7) CHA(7) CHA(7) Thickener 11 8 11 9 9 9 (mass %) Base oil (mixing Mineral Mineral PAO (100) Mineral Mineral Mineral ratio by mass) oil (100) oil (100) oil (100) oil (100) oil (100) Base oil (mass %) 89 92 89 91 89 89 Kinetic viscosity 40 100 100 100 100 100 of base oil (40° C.: mm²/s) Phosphoro- 2.0 thionate*⁷ Calcium 2.0 carbonate*⁸ Worked 300 340 300 300 300 300 consistency Abrasion wear Δ Δ x x ∘ x (25° C.) Abrasion wear x x x x x x (−30° C.) Corrosion x x x x x x preventive properties

TABLE 5 Comp. Comp. Comp. Comp. Comp. Comp. Ex. 13 Ex. 14 Ex. 15 Ex. 16 Ex. 17 Ex. 18 Isocyanate MDI*¹ MDI*¹ MDI*¹ MDI*¹ MDI*¹ Li soap Amine OA OA OA OA OA (molar ratio) Thickener 9 9 9 9 7 8 (mass %) Base oil (mixing Mineral Mineral Mineral Mineral Mineral Mineral ratio by mass) oil (100) oil (100) oil (100) oil (100) oil (100) oil (100) Base oil (mass %) 89 89 89 91 93 90 Kinetic viscosity 100 100 100 40 100 100 of base oil (40° C.: mm²/s) Zinc 2.0 naphthenate*² Zinc stearate*⁹ 2.0 MoDTC*¹⁰ 2.0 ZnDTP*¹¹ 2.0 Worked 300 300 300 300 340 300 consistency Abrasion wear ∘ x x Δ Δ x (25° C.) Abrasion wear x x x x x x (−30° C.) Corrosion x x x x x ∘ preventive properties *¹diphenylmethane diisocyanate *²metal salt of naphthenic acid mainly comprising zinc cyclopentanoate *³salt of a straight-chain fatty acid having eight carbon atoms and a straight-chain aliphatic amine having eight carbon atoms *⁴calcium dinonylnaphthalenesulfonate *⁵zinc dinonylnaphthalenesulfonate *⁶barium dinonylnaphthalenesulfonate *⁷mixture of alkylphosphoro thionate compound and amine-based compound, HITEC 833 (trade name, made by Afton Chemical Corporation) *⁸calcium carbonate *⁹zinc stearate *¹⁰MOLYVAN A (trade name, made by R. T. Vanderbilt Company, Inc.) *¹¹LUBRIZOL 1395 (trade name, made by The Lubrizol Corporation) ODA: octadecylamine CHA: cyclohexylamine OA: octylamine PAO: poly-α-olefin

In the above tables, the added amount, i.e., mass % with respect to the total mass of grease in parentheses of the items marked with *2 to *6, *8 and *9 represent the concentration of active ingredients in the additive.

In the above tables, the added amount, i.e., mass % with respect to the total mass of grease in parentheses of the items marked with *7, *10 and *11 represent the concentration of the added commercially available product.

The grease compositions of Examples 1 to 14 where the metal salt of naphthenic acid, salt of fatty acid having 6 to 10 carbon atoms and aliphatic amine having 6 to 10 carbon atoms, or metal salt of an organic sulfonic acid is added as the additive exhibit minimum abrasion wear and excellent anti-fretting properties without increasing the consistency under the conditions of ordinary temperature and the low temperature; and excellent corrosion preventive properties.

The grease compositions of Comparative Examples 1, 9 and 10 where no particular additive of the invention is contained show considerable abrasion wear and poor anti-fretting properties under the conditions of ordinary temperature and the low temperature; and poor corrosion preventive properties.

The grease compositions of Comparative Examples 2, 4, 11 and 13 where any additives different from the particular additive of the invention are contained show reduced abrasion wear at ordinary temperature, but considerable abrasion wear and poor anti-fretting properties at the low temperature, and inferior corrosion preventive properties.

The grease compositions of Comparative Examples 3, 5, 6, 12, 14 and 15 where any additives different from the particular additive of the invention are contained show considerable abrasion wear and poor anti-fretting properties under the conditions of ordinary temperature and the low temperature, and also poor corrosion preventive properties.

The grease compositions of Comparative Examples 7 and 16 employing the base oils with a low kinetic viscosity show slight abrasion wear at ordinary temperature, but considerable abrasion wear and poor anti-fretting properties at the low temperature, and also poor corrosion preventive properties.

The grease compositions of Comparative Examples 8 and 17 where the content of the thickener is decreased show a higher worked consistency and slight abrasion wear at ordinary temperature, but considerable abrasion wear and poor anti-fretting properties at the low temperature, and also poor corrosion preventive properties.

The grease composition of Comparative Example 18 employing the lithium soap as the thickener show considerable abrasion wear and poor anti-fretting properties at ordinary temperature and the low temperature although satisfactory corrosion preventive properties are found 

1. A grease composition comprising a base oil, a thickener and an additive, wherein; the base oil has a kinetic viscosity at 40° C. of 50 to 200 mm²/s; the thickener comprises a diurea compound represented by formula (I): R¹—NHCONH—R²—NHCONH—R³  (I) (wherein R² is a divalent aromatic hydrocarbon group having 6 to 15 carbon atoms, and R¹ and R³, which may be the same or different, represent cyclohexyl group or a straight-chain or branched alkyl group having 8 to 22 carbon atoms); and the additive is at least one selected from the group consisting of a metal salt of naphthenic acid, a salt of a fatty acid having 6 to 10 carbon atoms and an aliphatic amine having 6 to 10 carbon atoms, and a metal salt of an organic sulfonic acid.
 2. The grease composition of claim 1, wherein the metal salt of naphthenic acid is at least one selected from the group consisting of zinc cyclopentanoate and zinc cyclohexanoate; the amine salt is a salt of a straight-chain fatty acid having 6 to 10 carbon atoms and a straight-chain aliphatic amine having 6 to 10 carbon atoms; and the metal salt of an organic sulfonic acid is at least one selected from the group consisting of calcium dinonylnaphthalenesulfonate, zinc dinonylnaphthalenesulfonate and barium dinonylnaphthalenesulfonate.
 3. The grease composition of claim 1, wherein the metal salt of naphthenic acid is contained in an amount of 0.5 to 5.0 mass %, the amine salt is contained in an amount of 1.5 to 5.0 mass %, and the metal salt of an organic sulfonic acid is contained in an amount of 1.5 to 5.0 mass %, with respect to the total mass of the grease composition.
 4. A mechanical part in which the grease composition of claim 1 is packed. 